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It follows the police refusal to supply details about the amount and frequency of information provided, saying the release of details might lead to a reduction in the supply of financial data.

The Weekend Herald uncovered the arrangement, which appears to rely on a legal loophole which allows police to gain personal financial information without going through the court process. The loophole emerged in the Kim Dotcom case where information surrendered by his banks was used in United States courts to get orders to seize assets in New Zealand.

In other cases, it has been passed to Inland Revenue and the Ministry of Social Development.

It works through police simply asking for the information while referring banks to the Privacy Act section allowing confidentiality to be waived to aid "maintenance of the law".

Privacy expert and lawyer John Edwards said the "cosy relationship" left banks exposed to potential legal action from customers.

"They wouldn't be able to say the police asked for it so that's the end of it. If Kim Dotcom wanted to make an example of them, he could cause a great deal of trouble."

He said the banks could test the request by seeking more information from police - although he doubted police would want to disclose details.

He also questioned whether banks would be qualified to judge.

Mr Edwards said the community perceived financial information to be among the most private - "second only to health information in terms of sensitivity". He said the public could take some comfort from a formal "Memorandum of Understanding" which set out the process by which information requests were managed.

The Weekend Herald has found an agreement exists - but neither the police nor the NZ Banking Association will make it public.

The banking association's Philip van Dyk said: "It is not a public document and we won't release it. I can't comment on the agreement."

He said he had no knowledge of what checks the banks took to make sure the law was being maintained.

The association, which touts itself as being "the industry's voice", took customer confidentiality very seriously, he said.

The Privacy Commission has said banks need to form a "reasonable belief" and simply having a request from the police might not be enough.

Green Party human rights spokeswoman Jan Logie has called on the commission to go further and has written to it urging it to reassure the public on the issue.

She said the police should be using search warrants to gain access to people's banking information.